From the Terminal Norte bus station:

Fastest/Easiest: A taxi costs about $10.000 pesos (about $4.50 USD). Tell the driver to go to Parque Lleras and then to take Calle 7 up the hill two blocks to the hostel. Please keep in mind that all taxi drivers in Medellin are required to always use the Taxi meter.

Cheaper: By Metro/Bus combination. In the Caribe Metro Station by the Terminal Norte, buy a "Poblado Integrado" ticket: they will give you one ticket for the Metro and another for a Metro bus. Take the Metro south to the "El Poblado" Metro Station. Once there, catch a bus that says "Metro" - using your other ticket - to take you up the hill. Get off at Dominoes Pizza on Carrera 36. Walk down Carrera 36 to Calle 7 and you will see Casa Kiwi Hostel on the left.

From the Terminal Sur bus station AND the Olaya Herrera local airport:

Fastest/Easiest: A taxi costs about $6.000 pesos (about $2.50 USD). Tell the driver to go to Parque Lleras and then to take Calle 7 up the hill two blocks to the hostel. Please keep in mind that all taxi drivers in Medellin are required to always use the Taxi meter.

Cheaper: I do not recomend this. It is very far and like anywhere might be dangerous at night. If you really enjoy walking with a heavy backpack, it is possible to walk (about 45 minutes) from the Terminal Sur to the hostel. Follow Calle 10 all the way over the river/highway and then up the hill to Carrera 36 and Dominoes Pizza. Take a right on Carrera 36 and walk two blocks. Casa Kiwi Hostel will be on your left at the end of the street.

From the Rionegro/Jose Maria Cordova international airport:

Fastest/Easiest: A taxi costs about $55.000 pesos (about $24.00 USD). This is one of the few instances where there is a set taxi price, and the driver will not use the Taxi meter. Tell the driver to go to Parque Lleras in El Poblado and then to take Calle 7 up the hill two blocks to the hostel.

Cheaper: You can also take a "collectivo" van from the airport to the center of Medellin for about $10.000 pesos (about $4.50 USD), and then a taxi from the center to the hostel that should cost about $7.000 (about $3.00 USD). Tell the driver to go to Parque Lleras and then take Calle 7 up the hill two blocks to the hostel.